Two clinical psychopharmacological studies in children with conduct disorders are proposed: Study I: The comparative efficacy of methylphenidate, lithium, and placebo will be examined in children diagnosed as conduct disorders, using objective diagnostic criteria. 120 non-retarded children, (6-13) will be randomly assigned to treatment for 5 weeks. Clinical treatment effects will be examined on teacher and parent rating scales that include extensive sampling of ADDH and antisocial behaviors and on direct classroom behavior. Emanative treatment effects will be investigated by studying treatment induced changes in mother-child interaction. Finally, possible enhancing or deleterious effects of methylphenidate and lithium on cognitive and academic performance will be assessed to provide a comprehensive picture of the behavioral domains affected by each active treatment compared to placebo, and to each other. Study II will enter children who benefit significantly from short- term methylphenidate treatment into a one-year trial (estimated N = 90). They will be seen monthly, and reevaluated on all dependent measures used in Study I (behavioral, cognitive, academic, school observational, mother-child interaction, children self-ratings after 3, 6, 9, and 12 months). Children will randomly go on placebo or remain on methylphenidate, double-blind, after 6 months, and 12 months of methylphenidate. Assessments will be taken while the children are on methylphenidate and after the random assignment to placebo or continued methylphenidate. The studies will inform as to the efficacy of two very promising pharmacotherapeutic interventions, and to the relative long-term efficacy of a psychostimulant, a commonly used treatment.